The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for a boiling water reactor.
A fuel assembly used for the present boiling water reactor comprises a channel box and a fuel bundle of an 8.times.8 lattice which is housed therein, as is shown in FIG. 5 of the paper under the title "Present State and Future Perspectives of BWR Core-Fuel Design" by Toshiaki Enomoto, p.109-116, Journal of the Atomic Energy Society of Japan Vol. 26, No. 2 (1982). The fuel bundle is composed of fuel rods, water rods and special fuel rods which contain a fuel material with burnable poison having a large neutron absorption cross section such as Gd.sub.2 O.sub.3 added thereto. Such a special fuel rod will be referred to as "a fuel rod containing gadolinia" hereinunder.
In order to operate a nuclear reactor for a certain period, it is necessary for the reactor to have, at the initial stage of operation, an excess reactivity equivalent to the reactivity detracted by burning of fissile material in the reactor operation. On the other hand, it is necessary to control the excess reactivity in order to maintain the reactor in a critical state. For this purpose, a method of inserting a control rod of a neutron absorber into a reactor core has been adopted together with a method of using a fuel assembly having a fuel rod containing gadolinia.
The life time of a fuel assembly could be prolonged by increasing the enrichment of a fissile material in order to improve the fuel economy. However, the increase in enrichment brings about various problems. First, with the increase in enrichment, the excess reactivity which is to be controlled at the early stage of burning fuel is increased, which requires a larger number of fuel rods containing gadolinia than in the prior art. Furthermore, since the average energy of a neutron increases, the effect of each fuel rod containing gadolinia on controlling the excess reactivity reduces. Secondly, if the operational cycle length is to be prolonged with a prolonged life time of fuel, it is necessary to increase the rate of weight of gadolinia, which is unfavorable because it lowers the melting point of a fuel pellet. In addition, with the prolonged life time of fuel, the amount of fission product gas release from the fuel pellets of a fuel rod containing gadolinia becomes greater than that from the fuel pellets of an ordinary fuel rod, and the internal pressure of the fuel rod containing gadolinia increases. This is because the fuel rod containing gadolinia experiences the history wherein the power thereof rapidly increases with the process of burning and when the power of the fuel assembly is at its maximum, the power of the fuel rod also takes its maximum value. As a counter-measure for the increase in internal pressure, a method of reducing the diameter of a fuel pellet containing gadolinia seems to be effective, while the reduced diameter of a fuel pellet containing gadolinia is disclosed in Japanese Patent Laid-Open No. 153987/1979. This method, however, involves the reduction in amount of uranium to be loaded, which is disadvantageous from the viewpoint of fuel economy, and this method cannot solve the above-described problems.